1. The Field of the Invention
The present invention is related generally to thin film optical coatings for producing interference pigments. More specifically, the present invention is related to methods, systems and apparatus for producing thin interference coatings in the form of thin coalescence films which exhibit enhanced colorant or pigment effects and enhanced hiding power.
2. The Relevant Technology
Interference pigments and colorants have been used to provide a colored gloss in substances such as cosmetics, inks, coating materials, ornaments and ceramics. Typically, a silicatic material such as mica, talc or glass is coated with a material of high refractive index, such as a metal oxide, and a layer of metal particles is further deposited on top of such highly refractive material. Depending on the type and the content of the highly refractive material, different types of gloss and refractive colors are produced.
Thin film flakes having a preselected single color have been previously produced. For example, U.S. Pat. No. 4,434,010 discloses flakes composed of symmetrical layers that may be used in applications such as automotive paints and the like. The flakes are formed by depositing a semi-opaque metal layer upon a flexible web, followed by a dielectric layer, a metal reflecting layer, another dielectric layer, and finally another semi-opaque metal layer. The thin film layers are specifically ordered in a symmetric fashion such that the same intended color is achieved regardless of whether the flakes have one or the other lateral face directed towards the incident radiation.
High chroma interference platelets for use in paints, including color shifting and nonshifting single color platelets, are disclosed in U.S. Pat. No. 5,571,624. These platelets are formed from a symmetrical multilayer thin film structure in which a first semi-opaque layer such as chromium is formed on a substrate, with a first dielectric layer formed on the first semi-opaque layer. An opaque reflecting metal layer such as aluminum is formed on the first dielectric layer, followed by a second dielectric layer of the same material and thickness as the first dielectric layer. A second semi-opaque layer of the same material and thickness as the first semi-opaque layer is formed on the second dielectric layer. For the color shifting designs, the dielectric materials utilized, such as magnesium fluoride, have an index of refraction less than 2.0. For small shifting or nonshifting designs, the dielectric materials typically have an index of refraction greater than 2.0.
U.S. Pat. No. 5,116,664 discloses a pigment that is made by coating a first layer of TiO.sub.2 onto mica followed by coating the TiO.sub.2 layer with powder particles of at least one of the metals cobalt, nickel, copper, zinc, tin, gold, and silver. The metallic powder layer is deposited by an electroless wet chemical process to a thickness of 5 .ANG. to 1000 .ANG.. Electron microaraphs showed that these particles were in the form of finely divided rods.
U.S. Pat. No. 5,573,584 discloses a process for preparing forgery proof documents by printing with interference pigments. This patent discloses the process of overcoating platelet-like silicatic substrates (micas, talc or glass flakes) with a colorless or selectively absorbing metal oxide of high refractive index, a second, non-selectively absorbing, semitransparent layer, and optionally, a third layer comprising a colorless or selectively absorbing metal oxide in combination with scattering pigments. The second, non-selectively absorbing, semitransparent layer may be composed of carbon, metals, for example those which can be applied by gas phase decomposition of volatile compounds, such as compounds of iron, cobalt, nickel, chromium, molybdenum or tungsten, or metal oxides, such as iron oxide, magnetite, nickel oxide, cobalt oxides, vanadium oxides, or mixtures thereof.
Overcoating of a base material such as a TiO.sub.2 -coated silicatic substrate with an outer layer of carbon, metal or metal oxide is usually accomplished in conventional processes by chemical deposition methods such as electroless plating or pyrolysis methods. Electroless plating methods involve a redox process with no extraction or supply of electric current. Pyrolysis methods rely on the thermal decomposition of a volatile compound such as a hydrocarbon or an organometallic compound whose pyrolytic decomposition product is deposited on the surface to be coated.
Electroless deposition methods and pyrolytic methods, however, produce large islands or dots of the material being deposited on the base material. Consequently, continuous coating is obtained at the expense of depositing enough coating material to sufficiently coat the gaps between such large islands or dots. This extensive deposition leads in turn to a thick coating which, because of its thickness, does not generate the best chromatic colors. In short, these conventional methods produce thick coalescence layers.
When the preservation of the surface structure of the material that is being coated is desired, a thick coalescence layer has the disadvantageous feature of significantly altering such underlying surface structure. For example, photomicrographs of TiO.sub.2 -coated mica that were treated in an electroless cobalt plating bath have been reported as showing finely divided rod-like particles on the surface of the TiO.sub.2 layer. See, for example, U.S. Pat. No. 5,116,664, FIG. 1 and col. 6, lines 10-16, showing and describing a coating with finely divided rod-like particles.
The development of physical separation of columnar grains is not a fully understood process, and further examination of the factors that lead to a particular type of deposit formation need detailed examination. It has been reported in this respect that deposited films are in general in a state that ranges from that of being subject to tensile stresses to that of being subject to compressive stresses. See, Myong Ryeong Kim, Magnetron Sputter Deposited CoPtCr Magnetic Thin Films For Information Storage, at p. 73, Ph.D. Dissertation Thesis, Department of Metallurgical Engineering, The University of Utah, June 1993 (this Thesis will hereinafter be concisely referred to as "Magnetron Sputter Deposited Thin Films"). It has further been suggested in this regard that "a combination of . . . [a] residual stress contribution existing in thin film layers and a geometrical shadowing effect induces the development of physically separated columnar structure at high film thickness." Magnetron Sputter Deposited Thin Films, at p. 78. Nevertheless, "the exact mechanism involved in the residual stress effect is not yet clear." Magnetron Sputter Deposited Thin Films, at p. 80.
Chemical methods of deposition and electroless plating methods are typically limited to materials that involve hydrocarbons (liquid or gases), to organometallic compounds, and to metals, such as silver or nickel, that can readily be deposited by electroless processes. It is desirable, however, to manufacture mica interference pigments with methods that permit a much wider choice of materials. In particular, it is desirable to develop a process that can utilize materials, such as metals and sub-oxides, that can be vacuum deposited, materials, such as metal carbides, metal nitrides, metaloxynitrides, metal borides, and metal sub-oxides, that can reactively be deposited in vacuum, and materials, such as diamond-like carbon and amorphous carbon, that can be deposited by plasma-assisted vacuum methods.
Chemical methods of deposition and electroless plating methods typically generate solutions that must be disposed of, and some of these methods rely on catalytic substances that are incorporated into the pigments to an extent such that it prevents the use of the pigment in certain applications in various consumer products such as cosmetics. To avoid these problems and limitations, it is desirable to develop processes for manufacturing pigments that are more environmentally friendly and that do not rely on materials that can limit the use of the pigments. In particular, it is desirable to develop processes that reduce or eliminate the use of toxic materials and hazardous methods of deposition.
In addition to the need for developing processes for manufacturing interference pigments that can use a great variety of materials, it is also desirable to develop processes that can use cheaper materials and that permit the production of highly adherent and hard films that do not easily detach themselves from the substrate. In particular, it is desirable to provide processes that use materials other than the relatively expensive and mostly toxic metal carbonyls that typically require an investment in equipment for handling them and for monitoring their use in specially confined facilities.